(1) Field of the Invention
The present invention relates to a transistor made of a nitride semiconductor.
(2) Description of the Prior Art
In recent years, there has been actively made studies of field effect transistors (referred to as “FET” hereinafter) made of a GaN-based material, as high-frequency and high-power devices.
Hereinafter, a conventional transistor made of a nitride semiconductor material is described.
FIG. 1 shows a sectional view of a conventional FET having an AlGaN/GaN hetero junction. FIG. 2 shows distribution of fixed charges and free electrons induced by polarization in the conventional FET shown in FIG. 1. FIG. 3 shows an energy band diagram in the conventional FET. FIG. 4 shows a relation between a gate voltage and a drain current in the FET using two-dimensional electron gas as a carrier.
As shown in FIG. 1, the conventional FET made of a nitride semiconductor includes: a sapphire substrate 901 which has a (0001) plane as a main plane; an undoped GaN layer 902 which is formed on the sapphire substrate 901; an undoped Al0.25Ga0.75N layer 903 which is formed on the undoped GaN layer 902; a source electrode 905 and a drain electrode 906 which are formed on the undoped Al0.25Ga0.75N layer 903 and are made of Ti and Al; a gate electrode 907 which is formed on the undoped Al0.25Ga0.75N layer 903 and is made of Pd; and a passivation film 904 which covers the undoped Al0.25Ga0.75N layer 903 and is made of SiN.
In the conventional FET shown in FIG. 1, two-dimensional electron gas of about 1×1013 cm−2 is generated at a hetero junction interface between the undoped GaN layer 902 and the undoped Al0.25Ga0.75N layer 903, irrespective of no implantation of impurities, due to spontaneous polarization and piezoelectric polarization inherent in materials of the undoped Al0.25Ga0.75N layer 903.
As shown in FIG. 2, negative fixed charges are generated at a top face of the undoped Al0.25Ga0.75N layer 903 (a face contacting with the gate electrode 907) and a top face of the undoped GaN layer 902, respectively. On the other hand, positive fixed charges are generated at a bottom face of the undoped Al0.25Ga0.75N layer 903 (a face closer to the sapphire substrate 901) and a bottom face of the undoped GaN layer 902, respectively. Herein, an absolute value of an amount of the fixed charges generated at the surface of the undoped Al0.25Ga0.75N layer 903 is higher than an absolute value of an amount of the fixed charges generated at the surface of the undoped GaN layer 902. Therefore, sheet carriers are generated in the form of two-dimensional electron gas at the hetero interface on the undoped GaN layer 902 side in an amount for compensating a difference between such fixed charges (Ns in FIG. 2). In FIG. 2, a solid arrow indicates the fixed charges generated at the undoped Al0.25Ga0.75N layer 903, and a broken arrow indicates the fixed charges generated at the undoped GaN layer 902.
With this polarization, an electric field is generated in the undoped GaN layer 902 and the undoped Al0.25Ga0.75N layer 903, and the energy band diagram is as shown in FIG. 3. In the undoped GaN layer 902, that is, a valence band in the vicinity of the hetero interface has a potential energy which is equal to or less than a Fermi level. Therefore, the conventional FET has an electric characteristic of a normally-on type as shown in FIG. 4.
The source electrode 905 and the drain electrode 906 contact with the undoped Al0.25Ga0.75N layer 903. In the case where the undoped Al0.25Ga0.75N layer 903 has a small thickness, for example, equal to or less than 30 nm, a channel region in which the two-dimensional electron gas is generated (herein, part of the undoped GaN layer 902) is electrically connected to the source electrode 905 and the drain electrode 906 by a tunnel current. Thus, the source electrode 905 and the drain electrode 906 serve as preferable ohmic electrodes. The gate electrode 907 made of Pd has a large work function of 5.1 eV, and achieves a preferable Schottky junction with the undoped Al0.25Ga0.75N layer 903 (refer to M. Hikita et al., Technical Digest of 2004 International Electron Devices Meeting (2004) p. 803).
In the case where a normally-off characteristic is realized using a GaN-based semiconductor material having polarization, as described above, it is necessary to reduce carriers generated in the channel by spontaneous polarization and piezoelectric polarization inherent in crystal. As for a FET having a hetero junction between AlGaN and GaN, if an Al composition in an AlGaN layer is lowered, a stress due to a difference in lattice constant between AlGaN and GaN is also reduced; thus, piezoelectric polarization is decreased. Consequently, a sheet carrier concentration is decreased (refer to O. Ambacher et al., J. Appl. Phys. Vol. 85 (1999) p. 3222). In the undoped Al0.25Ga0.75N layer 903, specifically, when a mixed crystal ratio of Al to Ga is lowered to 0.15 while a thickness is maintained at 30 nm, a sheet carrier concentration is considerably decreased from 1.4×1013 cm−2 to 5×1012 cm−2. This decrease in carrier concentration reduces an operating current. In addition, when the Al composition in the undoped Al0.25Ga0.75N layer 903 is lowered, a potential barrier at a gate is lowered.
Moreover, a forward voltage applicable to the gate electrode 907 has an upper limit value to suppress generation of a leakage current at the gate electrode 907. This hinders a gate voltage from becoming high, and makes it difficult to obtain a sufficiently large drain current.
In order to solve such problem, there has also been proposed a transistor having the following configuration. That is, a p-type doped gate is formed to enhance a potential barrier such that the transistor is of a normally-off type and can be applied with a high forward voltage. This transistor is referred to as a junction FET (JFET). Such JFET is described in L. Zhang et al., IEEE Transactions on Electron Devices, Vol. 47, No. 3, pp. 507-511, 2000 and Japanese Laid-Open Patent Application No. 2004-273486.
However, the JFET has a problem of a low switching speed due to holes accumulated in a channel region.